Mechanical face seals are used on various types of machines and equipment, such as pumps, compressors and gear boxes, which have rotating shafts and a sealing chamber adjacent the shaft wherein a fluid in the sealing chamber is prevented from leaking along the shaft. Such mechanical seals include a pair of adjacent seal rings having opposing seal faces which define a sealing region therebetween. One of these seal rings typically is mounted on the shaft so as to rotate therewith while the other seal ring is non-rotatably mounted on a seal housing. The fluid being sealed is disposed on one edge of the sealing region, wherein the opposing seal faces at least reduce leakage of the sealed fluid across the sealing region.
Most liquid seals operate with the seal faces rotating in contact. However, due to asperities in the seal faces, some leakage may occur. In gas seals, the seal faces often are provided with grooves or recesses to generate hydrodynamic lifting forces. In this regard, the seal faces typically separate a small distance wherein a thin film of fluid forms between the seal faces to lubricate the seal faces and reduce wear therebetween. Additionally, the grooves or recesses may pump the fluid film toward the fluid being sealed to reduce leakage of the sealed fluid.
With respect to specific constructions of mechanical seals, one representative mechanical seal is disclosed in U.S. Pat. No. 6,446,976 (Key et al), the disclosure of which is incorporated herein in its entirety by reference. In this mechanical seal, one of the seal faces includes a plurality of concentric shallow annular grooves which preferably are disposed near the seal ring diameter that is farthest away from the fluid being sealed. In general, the basic construction of mechanical seals and the use of relatively rotatable seal rings are well known, and a detailed discussion of such mechanical seals is not required herein.
More particularly, dry running lift-off face seals, also called fluid film, gap, or non-contacting face seals, have found application in both gas and liquid sealing applications in compressors and pumps. The fluid film between the seal faces allows the seal to operate with minimum heat generation and no wear.
Dry running lift-off face seals utilize a variety of shapes of shallow grooves to create lift between the seal faces, allowing the faces to run without contact. Existing examples include spiral grooves, radially tapered waves, and T-grooves. These various grooves are designed to provide a varying combination of hydrostatic and hydrodynamic load support to achieve separation of the seal faces by a small gap. Hydrostatic load support is created through the manipulation of the fluid pressures acting between the seal faces, and is not dependant on motion between the seal faces to create lift. Hydrodynamic load support is created through the active compression of the fluid between the seal faces due to movement of the fluid from a wide gap to a narrower gap, and requires relative motion between the seal faces to create lift. This relative motion typically occurs during shaft rotation.
The geometry of the shallow grooves determines the amount of hydrostatic and hydrodynamic load support created at a given set of operating parameters. The total load support provided must be in equilibrium with the pressure and mechanical forces that act to close the seal faces at a specified operating gap.
One specific use for a mechanical seal is in boiler feed water applications in power plants, wherein the mechanical seal may be used to seal the rotating shaft of a pump by which the boiler feed water is being pumped. One unique problem associated with this application is that the feed water conditions generate electrical type corrosion of the seal faces. Such feed water may have a water chemistry which results in low conductivity, typically less than 2 uS (microsiemen). These conditions result in corrosion of the mechanical seal materials from which the seal rings are formed, which materials commonly can include silicon carbide (SiC) and tungsten carbide. The observed corrosion of the seal ring materials can be attributed to the electrically insulating properties associated with ultra-pure water chemistry. The damage commonly occurs at the outside diameter circumferential edges, drive slots, and high rotational velocity surfaces of the seal rings.
Several theories have been proposed as to this corrosion phenomenon such as erosion cavitation and Zeta spin potential with extensive laboratory testing conducted for verification. In attempting to overcome this problem, testing was conducted wherein grades of silicon carbide where altered to change electrical resistance but this did not prevent electrical corrosion in laboratory testing. In the alternative, chemical vapor deposition (CVD) coatings were investigated as a potential means of preventing corrosion in ultra-pure water applications. Various coatings were laboratory tested with marginal results due to application uniformity and localized pitting.
The objective of this invention is to provide an improved mechanical seal and seal ring construction which overcomes the problems associated with electrical corrosion in certain applications and particularly, ultra-pure water applications.
The invention relates to a unique seal ring construction for mechanical face seals, wherein the mechanical seal ring is coated over critical surfaces with a tantalum coating. The coating preferably is applied to at least some of the seal ring surfaces which are exposed to the process fluid so as to at least minimize electrical corrosion. The tantalum is applied in an inventive method such that the tantalum is applied to a carbide, and preferably silicon carbide (SiC) seal ring through chemical vapor deposition (CVD). The CVD process of applying the tantalum is conducted at an elevated temperature that adequately effects a reaction between the tantalum coating and the SiC substrate such that the tantalum and SiC form an intermediate transformation layer of tantalum carbide between an outer surface layer of tantalum and the SiC substrate. The transformation layer of tantalum carbide provides a strong bond between the tantalum layer and SiC substrate.
If desired, the outer tantalum layer on the seal face can then be machined, such as by laser machining, to form hydrodynamic face patterns at a depth less than the thickness of the tantalum coating layer. In this regard, it is preferred that the depth of the face pattern does not extend into the intermediate layer such that the interior surfaces of the face pattern are still comprised of tantalum.
In a first aspect, the invention relates to the structure of a seal ring formed of a SiC substrate, a tantalum carbide transformation layer, and a tantalum surface layer wherein the tantalum carbide forms during the application of the tantalum. In a second aspect, the invention relates to the method for forming this seal ring through a CVD process which supplies tantalum to the SiC substrate wherein an intermediate transformation layer forms during the CVD process.
Other objects and purposes of the invention, and variations thereof, will be apparent upon reading the following specification and inspecting the accompanying drawings.
Certain terminology will be used in the following description for convenience and reference only, and will not be limiting. For example, the words “upwardly”, “downwardly”, “rightwardly” and “leftwardly” will refer to directions in the drawings to which reference is made. The words “inwardly” and “outwardly” will refer to directions toward and away from, respectively, the geometric center of the arrangement and designated parts thereof. Said terminology will include the words specifically mentioned, derivatives thereof, and words of similar import.